One type of image sensor is an active pixel sensor (APS). APS image sensors are typically fabricated using metal oxide semiconductor (MOS) processing technology, in particular for example Complementary Metal Oxide Semiconductor (CMOS) processing technology, and are typically referred to as (C)MOS image sensors. CMOS image sensors sense light by converting incident radiation (photons) into electronic charge (electrons) via the photoelectric effect. CMOS image sensors typically include a photoreceptor (e.g. photodiode) and several CMOS transistors for each pixel.
Existing CMOS image sensors include, but are not limited to, three-transistor (3T) and four-transistor (4T) pixel implementations. Pixel implementations with more than four transistors have also been implemented. The pixel circuits in this type of image sensors typically include a source follower transistor that is used to buffer the photoreceptor voltage onto a column line.
An existing pixel configuration is illustrated in FIG. 1. FIG. 1 illustrates, as an example only, a 4T pixel 10 for a CMOS image sensor. All transistors in the pixel are MOS transistors. The pixel 10 illustrated in FIG. 1 comprises a photoreceptor 11, in the example illustrated a pinned photodiode PPD, for converting impinging radiation into electronic charge. The pixel 10 furthermore includes a sample and hold transistor 12 for transferring charge generated by the photoreceptor 11 towards a sense node 16, a reset transistor 13 for resetting the sense node to a starting value, a source follower transistor 14 for converting the transferred charge into a voltage and a select transistor 15 for actually putting that voltage onto a column line.
A classical configuration of a CMOS image sensor 20 is illustrated in FIG. 2. The image sensor 20 comprises a 2-dimensional array 21 of pixels 10 arranged in n rows and m columns. In the example illustrated in FIG. 2 there are 7 rows and 10 columns. The pixels 10 in the array may be any suitable type of pixels, for examples pixels as illustrated in FIG. 1.
In typical prior art systems, all pixels 10 arranged on a column in the array 21 are connected to a column bus OUT for reading out pixel information generated by and stored in the pixels 10. Pixel information from a CMOS image sensor is typically read out sequentially, row per row. This sequential readout technique is realized by horizontal and vertical scan circuitry. These digital parts are not illustrated in FIG. 2.
The prior art readout process can start at the top, at the bottom of the array, or at any randomly chosen row, but in any case rows are read out one after the other. This means that first the pixel information of pixels on a first row is read out, thereafter pixel information of pixels of a second row, typically adjacent the first row, and so on.
A classical timing that goes together with such a way of reading the sensor may be as follows (based on the pinned photodiode implementation with four transistors in each pixel, as illustrated in FIG. 1):                a) All pixels in a particular row are being reset by means of reset transistor 13;        b) The select transistor 15 is activated, and the source follower 14 is connected to a bias current;        c) The reset level is sampled by the analog circuitry 22 of the columns;        d) The sample and hold transistor 12 is activated and the electrons are transferred from the photoreceptor 11 towards the sense node;        e) The actual video level is sampled by the analog circuitry 22 of the columns;        f) When the two samples (reset level and actual video level) are available, they are subtracted from one another in the analog domain;        g) The analog signal is converted into the digital domain by means of AD converters 23;        h) The digital data is multiplexed and transferred off chip via digital output bus 24.        
The complete cycle from a) to h) is required for reading out pixel information from a single line of pixels, and will take up a complete line time. With a simple calculation, taking as an example a sensor with 6000 columns and 4000 rows, 5 frames/s, the total frame time will be 200 ms. The total available line time will be: 1/(4000*5)=0.05 ms.
In case one is not interested in the complete information from the sensor, but only in the information of one column, still the complete sensor needs to be read out because the readout mechanism is based on a line-by-line system. In the aforementioned case, it will take 200 ms to read out pixel information from a single column of pixels.